The present invention relates to a control system for controlling power transmission through a power transmission system that is capable of performing an engine-idling elimination control when the vehicle comes into a halt. Furthermore, the present invention relates particularly to a power-transmission control system characterized in a control that restarts the vehicle by engaging a starting clutch after restarting the engine, which has stopped in the idling elimination control.
Generally, automobiles include a construction for transmitting the driving force of an engine to wheels through a power transmission. Additionally, for the purpose of improving fuel economy, many vehicles nowadays are equipped with an idling elimination control, in which the operation of the engine is stopped when the vehicle comes into a halt (refer to, for example, Japanese Patent Publication No. 3011069 and Japanese Laid-Open Patent Publication No. 2000-272380).
Generally, a power transmission comprises a hydraulically operated frictionally engaging element (hydraulic clutch, hydraulic brake, etc.), which is used to control the transmission of the driving force of an engine to wheels. In a case where an idling elimination control is executed with the power transmission, when the operation of the engine is stopped, the hydraulic pump that supplies a hydraulic pressure to the frictionally engaging element stops, and therefore, the frictionally engaging element itself also stops because of the construction of the transmission, in which the pump is driven by the engine. For starting the vehicle, when the engine is restarted from the stopped condition in the idling elimination control, the engine being started drives the pump again, which supplies the hydraulic pressure to frictionally engaging element. Here, the start of the engagement of the frictionally engaging element follows the start of the engine. Because of the delay of the engagement of the frictionally engaging element with respect to the starting of the engine, immediately after the restarting of the engine and before the real engagement of the frictionally engaging element, an engine racing can occur, which impairs the driving performance of the vehicle.
To solve this problem, the power transmission system disclosed in Japanese Patent Publication No. 3011069 includes electricity-generating electrical motor means (electrical motor generator). For starting the vehicle, when the engine is restarted from the stopped condition in the idling elimination control, the electrical motor generator is actuated to load the engine (to generate electricity), thereby preventing any occurrence of engine racing. Also, the system disclosed in Japanese Laid-Open Patent Publication No. 2000-272380 is equipped with a control to restrain the output of the engine when the engine is restarted from the stopped condition in the idling elimination control until the engagement of the frictionally engaging element starts. Specifically, in this control, the throttle of the engine is kept closed to restrain fuel supply for prevention of engine racing.
However, in the system disclosed in Japanese Patent Publication No. 3011069, if the vehicle in the idling elimination control were stopped and started repeatedly, the electrical motor generator would heat up or could even overheat. Such an action can reduce the durability of the motor generator. Therefore, if the electrical motor generator heats up, then a safety device is actuated to prohibit the idling elimination control. This results in a reduced fuel efficiency. By the way, the electrical motor generator is to assist the driving force of the engine when the vehicle is started. If the electrical motor generator is used to prevent engine racing as described above, then the electrical motor generator must be controlled to prevent engine racing and then to assist the engine. However, it is difficult to determine a correct or right timing for the operation of the electrical motor generator to be switched from the former aspect to the latter in this control. If this timing is not right and delayed, then the driving performance of the vehicle is impaired.
On the other hand, the system disclosed in the above mentioned Japanese Laid-Open Patent Publication No. 2000-272380 requires a controller that controls the engine throttle independently from the operation of the accelerator pedal. Because of this requirement, the engine control system is complicated. In this system, it is necessary to quickly bring the throttle opening of the engine to a degree that corresponds to the operation of the accelerator pedal immediately after the start of the engagement of the frictionally engaging element. As a result, the driver may feel that something is not right.
To solve the above mentioned problems, it is an object of the present invention to provide a power-transmission control system which enables a vehicle to start smoothly without engine racing after the engine has been stopped in an idling elimination control.
To achieve this objective, the present invention provides a vehicular power-transmission control system which comprises an engine, a drive-power transmission (for example, the belt-type continuously variable transmission CVT described in the following embodiment), a hydraulically actuated frictionally engaging element (for example, the starting clutch 5, the forward clutch 25 and the reverse brake 27 described in the following embodiment), an electrical motor (for example, the electrical motor generator M described in the following embodiment) and a hydraulic pump. The engine is controlled to stop when the vehicle comes into a halt in an idling elimination control, and the drive-power transmission transmits a rotational driving force from the engine to wheels. The hydraulically actuated frictionally engaging element controls the transmission of the rotational driving force in the drive-power transmission, and the electrical motor is provided closer to the engine than to the frictionally engaging element and can assist the rotational drive of the engine. The hydraulic pump is connected to the output shaft of the engine, and it is to supply an engagement-actuating pressure to the frictionally engaging element. The power-transmission control system performs the following control to restart the vehicle, which has come into a halt with the engine consequently stopped in the idling elimination control. The control system makes the electrical motor to rotate the output shaft of the engine, to which fuel supply is not allowed in this instance. By the rotation of the output shaft driven by the electrical motor, the hydraulic pump is operated to supply a hydraulic pressure to the frictionally engaging element. When the frictionally engaging element receiving the hydraulic pressure becomes ready for a transmission of power, the control system starts the operation of the engine, so that the engine can produce a torque. This control for starting the operation of the engine to produce a torque means either a control in which the engine is started when the frictionally engaging element has become ready for a power transmission, or a control in which the engine is started a little before the frictionally engaging element is ready for a power transmission, and when the frictionally engaging element becomes ready for a power transmission, the engine, which has already started, can provide a real driving torque.
According to a feature of this power-transmission control system, when the vehicle, whose engine has been stopped in the idling elimination control, is to be restarted, at first, the electrical motor drives the hydraulic pump, which is connected to the output shaft of the engine, which is not allowed to receive any fuel supply in this instance. In this condition, the frictionally engaging element receives oil discharged from the pump, and when the frictionally engaging element starts its engagement, the engine is started. After the frictionally engaging element has become ready for a power transmission (i.e., it has just started its engagement), the output of the engine is transmitted through the frictionally engaging element to the wheels to start the vehicle. In this way, the vehicle can be started smoothly without any engine racing. In this case, the electrical motor is used only for driving the hydraulic pump and for rotating the engine idly, so there is no possibility of the electrical motor being heated up excessively. Therefore, this electrical motor can be miniaturized in the design. In addition, if the electrical motor is kept operating even after the start of the engine, then the electrical motor can be controlled to assist the engine for driving the vehicle. This is advantageous for improving the driving performance of the vehicle. As no fuel is supplied in the initial stage of the control, this fact contributes to a fuel efficiency improvement.
Also, the suction and exhaust valves of the cylinders of the engine may be constructed to be capable of being kept closed. With this arrangement, when the vehicle, which has come into a halt with the engine consequently stopped in the idling elimination control, is to be restarted, while the hydraulic pump is being driven by the electrical motor in operation (i.e., while no fuel is being supplied to the engine), preferably, at least some of the suction and exhaust valves are kept closed. As a result, while the output shaft of the engine is being rotated by the electrical motor (while the engine is rotated idly), no or little fresh air enters the cylinders. This arrangement helps the exhaust gas to be kept clean at the time when the engine is activated. Furthermore, by keeping the suction and exhaust valves closed, the pumping loss, which would occur otherwise at a large degree if the air were allowed to flow through the passages of the suction and exhaust valves as the pistons reciprocate in the cylinders, is maintained at a small degree. This reduced loss leads to a reduced electricity consumption as the driving torque required of the electrical motor becomes small. In addition, because there is no opening and closing of the suction and exhaust valves, the vibration of the engine is comparatively small.
According to another feature of the present invention, another vehicular power-transmission control system comprises an engine, which can be controlled to stop when the vehicle comes into a halt in an idling elimination control, a drive-power transmission, which transmits a rotational driving force from the engine to wheels, a hydraulically actuated frictionally engaging element, which controls the transmission of the rotational driving force in the drive-power transmission, and a hydraulic pump, which is driven by the engine to supply an engagement-actuating pressure to the frictionally engaging element. This power-transmission control system performs the following control to restart the vehicle, which has come into a halt with the engine consequently stopped in the idling elimination control. The control system makes the engine to start in a condition where the output of the engine is reduced by an ignition-timing shift. After the frictionally engaging element receiving the hydraulic pressure from the hydraulic pump becomes ready for a transmission of power, and when the frictionally engaging element starts its engagement operation, the ignition timing of the engine is reset to a normal condition.
According to this power-transmission control system, to start the engine, which has been stopped in the idling elimination control, for restarting the vehicle, at first, the engine is started in an ignition-timing shift (for example, the ignition timing is delayed) for limiting the output of the engine. As a result, the engine starts without any engine racing, and the hydraulic pump being driven by the engine operated in this condition discharges oil, which is supplied to the frictionally engaging element. When the frictionally engaging element receiving the oil starts its engagement, the ignition timing of the engine is reset to a normal setting to increase the output of the engine to a normal level, so that the vehicle can be started smoothly. In this case, the rotational speed of the engine is controlled in correspondence to the operation of the accelerator pedal, so the driver feels no abnormality from the operation.
In this power-transmission control system, an electrical generator may be provided to the engine rather than to the frictionally engaging element, so that the electrical generator can be driven by the engine to generate electricity. While the engine is being operated in the ignition-timing shift as described above, the electrical generator can be driven by the engine to generate electricity. With this arrangement, as the engine having started in the ignition-timing shift has a reduced output, and as the engine in this condition is further restrained by the load of the electrical generator to generate electricity, the starting of the engine is securely protected from engine racing. In this case, when the frictionally engaging element receiving the oil discharged from the hydraulic pump actually starts its engagement, the ignition timing of the engine is reset to the normal setting, and the generation of electricity by the electrical generator is terminated. At this time, the output of the engine is reset to a normal level, so that the vehicle can be controlled to start smoothly. Also, in this case, the rotational speed of the engine is controlled in correspondence to the operation of the accelerator pedal, so the driver feels no abnormality. Furthermore, because the electrical generator is driven only by the engine producing a reduced output, the electrical generator never heats up excessively, so the switching of the electrical generator for the termination of electricity generation is performed smoothly.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.